Chemical nickel and cobalt plating process



Jan. 27, 1959 Filed May 20, 1955 SEWAGE OUTLET s. A. HAYS CHEMICALNICKEL AND COBALT PLATING PROCESS 2 Sheets-Sheet 1 COLUMN DRAINAGESEWAGE OUTLET FILTER SOLUTION 53 PLATING v OUTLET E E m 3 g m IL a:

SOLUTION PLATING INLET FlG.l

SEWAGE INVENTOR.

STEPHEN A. HAYS ATTORNEY 1959 s. A. HAYS 2,871,142

CHEMICAL NICKEL AND COBALT PLATING PROCESS Filed May 20, 1955 2Sheets-Sheet 2 53 52 43 PLATING SEWAGE SOLUTION y OUTLET PLATING-/- gTANK TAP WATER PLATING SOLUTION E v 50 OUTLET TAP V TAP WATER 3| WATER29 29 FIG.8

SEWAGE v SEWAGE Q SEWAGE TAP 3 TAP V v 39 WATER 29 WATER 3 FIG. I0 I 29FIG. 9 T INVENTOR.

STEPHEN A. HAYS ATTORNEY United States Patent Q CI-IEMICAL NICKEL ANDCOBALT PLATING PROCESS Stephen A. Hays, Sierra Madre, Calif., assignorto North American Aviation, Inc.

Application May 20, 1-955,SerialJNo. 509,808

12 Claims. (Cl. 117-102) The present invention concerns a new and novelprocess of chemical nickel or cobalt plating either in a batch orcontinuous type of operation. More particularly, the invention, involvesa method of maintaining the nickel or cobalt concentrations of theplating bath and of removing detrimental ions from the spent platingsolution.

In U. S. Patent No. 2,532,283, Brenner and Riddell describe a batchprocess of chemical nickel plating in which a nickel salt (preferablynickel chloride), a hypophosphite (sodium) and a buffer salt (sodiumsalts of carboxylic acids or ammonium salts) make up the platingsolution. The process described therein, while not expressing theoptimtun conditions and operating solution, forms the basis for thechemical nickel processes presently in use. Batch processes inherentlypresent many problems when their use extends to production work. Thedifiiculty of keeping within predetermined concentration limits andratios, accurate pH control, and removal of various undesirable ions andby-products are all apparent in batch type operation. These abovedifliculties result in variations in plate quality and limit the.thickness which can be continuously plated. Brenner et al., in U. S.Patent No. 2,532,284, show cobalt plating wherein cobalt chloride,sodium hypophosphite and sodium citrate are reacted with water toproduce cobalt. The above difiiculties are also present in cobaltplating.

In U. S. Patent No. 2,658,839, Talmey and Crehan describe a process ofnickel plating using a plating bath and a reservoir, the lattercontaining the bulk of the plating solution. T he reservoir ideaprovides a continuous process only by providing a much larger volume ofsolution, which takes longer for complete use. The reservoir concept hasthe advantage of storing the bulk of the solution at a lowertemperature, thus preventing thermal decomposition. in contradiction tothe process disclosed herein, the Talmey et al., process does notinclude any provision for actually regenerating the plating solution.Other prior attempts to provide a continuous system of chemical nickelor cobalt plating have all failed to prevent the gradual build-up ofdetrimental products in the plating solution.

The present invention involves, basically, the provision of a new andnovel process which includes providing a cation exchange column in thechemical nickel or cobalt plating process. The incorporation of theimprovements of the present invention prevent rapid build-up ofundesirable ion concentrations and precipitates in the plating solution.The efficiency of the present invention is predicated individually andcollectively on the extraction properties of cation exchange solids, onthe extraction properties of the chelatingor complexing anions insolution, on the counter-current extraction and on the inherent2,571,142 Patented Jan. 27, 1959 ICC? 2 close control of the complexing:agent-to-nickel (or co: balt) ratio in the plating bath.

The invention as hereinafter described willbe explained in terms ofchemical nickelplating. The discussion thereunder is also, applicable tocobalt plating. In the pre ferred form of the invention, the cationexchange resin is a nickel complexing agent in the solid state, whichwill extract nickel from an agueous sulfate solution. The fact that theresinv is a solid makes it possible to readily separate. it. Flowthrough a column of adequate length (equivalent to multiple "batchextractions) provides over 99.9% efiiciency. For example, citrate ion isa strongly competing nickel complexing. agent, such that an equilibriumdistribution of nickel between the solid resin and they dissolvedcitrate ion; would be achieved in. a batch extraction. However, flow ofthe citrate solution through a column in the opposite; direction to theflow of nickel sulfate solution used to: charge the column with nickel,may also be used to produce over 99.9% efficiency in either of two ways:(1) interms of citrate saturation when adequate nickel is present, or(2) in terms of complete removal of nickel when adequate citrate, passesthrough. The preferred form of the. present process utilizes the formeradequate nick elalthough the nickel content and column length need be nomore than Sufiicient to provide a nickel concentration in the emergingsoluio eq vale t ab u o h trat q sc t w tion, A he ame me. h concent aon f tion contaminants in the solution, such as, cadmium or calcium, isntinuou d srs fiin thr u he o umn. en th s that they are elficientlyremoved, avoiding inhibition of plating and precipitation of calciumsulfate when the solution is heated; Qt course, since these are allexchange reactions, addition ot nickel in this manner avoids addition ofthe accompanying anion (such as suliate) and removes an equivalentamount of other cations. This provides additional savings b y reducingthe build-up of total solids in the solution so that it is lessfrequently necessary to separate the solids or discard the bath.

The chemical reactions and steps involved in the preferred form of theinvention are 'as follows, where R represents the cation exchange resinnegative radical, and underlining denotes the solid state as dilferentiated from the solution state: i i

(1) Charging the cation exchange column with nickel:

(2) Charging the plating solution with nickel by passing the solutionthrough the cation exchange column:

2Na Citrate+SNiR Ni (Citrate) +6NaR (3) The plating reaction in which Niis the plated nickel:

Nlg (Citrate) 2 I 2H3 Citrate 3 NaH2P 03 mov ng impurities of a n s luion n the cation exchange column;

CdSO4+NiR2+ CdR +NiSO ca, (Citrateh-l-fiefifi-l-Ni citrate m v ng exce sac d rom p n olution:

2H3 Citrate+3NiR 6EIi-|-Ni (Citrate) The excess acid removal in (5),which exchanges hy drogen ions formed in the plating solution, occursalong with the exchange of sodium and nickel ions in the 'column. Suchhydrogen ion exchange varies inversely with respect to the pH and theamount of sodium ions present.

The incorporation of a cation exchanger in the electroless nickelprocess has many advantages. Among these is that it removes traces ofbath'inhibiting metal ions, such as cadmium. Further, it provides asource for introducing nickel to the bath in which both the anions, suchas sulfate, and an equivalent amount of undesirable sodium ions areeliminated from the plating solution. The process also provides for anautomatic control of the nickel-to-citrate ratio, thus avoiding frequentanalysis and simplifying solution maintenance. The presence of thecation exchange column, in addition, provides a convenient apparatus forremoving calcium, magnesium and other detrimental ions from the mainwater supply, thus permitting its substitution for the relativelyexpensive distilled water heretofore used in the electroless nickelprocess. As hereinafter explained, the column can also be used forrecovering nickel from waste.

An object of this invention is to provide a new electroless nickel andcobalt plating process.

A further object of this invention is to provide a new process ofchemical nickel plating.

A still further object of this invention is to provide an ion exchangecolumn through which an electroless nickel or cobalt plating solutioncirculates.

An additional object of this invention is to provide a method ofproviding automatic control of the citrate-tonickel ratio in acitrate-containing nickel plating bath.

A further object of this invention is to provide a continuous process ofregenerating the plating solution used in electroless nickel or cobaltplating.

A still further object of this invention is to provide a process ofchemically depositing nickel to form a tough, hard and adherent depositon various metallic surfaces.

An additional object of this invention is to provide a process for theremoval of calcium and cadmium inhibiting ions from an electrolessnickel or cobalt plating bath.

Other objects of invention will become apparent from the followingdescription taken in connection with the accompanying drawings, in whichFig. 1 represents the over-all flow sheet;

And Figs. 2 to ll illustrate the individual steps of the process indetail.

In the following example, the plating solution is assumed to containnickel citrate and sodium hypophosphite plus water which reacts to formcitric acid, sodium phosphite and nickel metal, as shown in Equation 3above. The process of this invention serves to regenerate the nickelcitrate or other nickel salt present in the plating solution. In Fig. 1,a cation exchange material, typically a resin in the form of sodiumsalt, is placed (as is well known in the water softening art) in an ionexchange column 50 and in two other columns 40 and 30. A tank 37 isprovided to dissolve the salts used to recharge these columns. Since thecolumns should be kept full of Water or solution, the outletshereinafter mentioned should open between the tops of the columns andthe bottom of the tank 37. In addition to the various valves shown it isto be understood that each of the columns will ordinarily have drainvalves to provide for emptying the columns when required. The processcomprises three major operations: first, charging the column 50 withnickel ions: second, treating the plating solution; and third, chargingthe columns and with sodium ions. Insofar as the electroless nickelplating process is concerned, the second step of treating the platingsolution is the essence of the present invention. The description of theremaining flow lines, valves and auxiliary equipment shown in Fig. 1will be explained in detail in considering the various specific steps ofthe process as individually illustrated in Figs. 2 to 11.

The first step in charging column 50 with nickel is to pass tap waterthrough valve 29, flow line 31, and forward through column 30 into tank37 to dissolve nickel sulfate in such tank. A moderately concentratedsolution (200-700 grams/liter of NiSo -6H O) is formed in tank 37. Inthis operation, valves 28 and 34 are open and valve 35 is closed. Column30 contains a cation exchange material which softens the incoming waterby removing such foreign cations as calcium and magnesium therefrom. Inthe next charging step the nickel sulfate solution from tank 37 ispassed slowly through Valve 34 (valve 28 being closed), through line 44,and through column 50 in reverse. The amount of nickel added should belimited so that to of the nickel added will be held by column 50. Thenickel sulfate solution flows through column 50, in reverse throughcolumn 40 by means of line 43, leaving any remaining nickel in column40. When the other bath cations are sodium, the nickel ions will beexchanged for sodium ions and the efiluent will emerge as a sodiumsulfate solution which may be discarded to Waste through line 39 andvalves 13 and 36. Preferably, a transparent section is provided betweencolumns 50 and 40 so that the nickel addition from tank 37 may bestopped when the usual green color of the nickel solution is seenpassing from column 50 to column 40.

Optionally, as seen in Fig. 4, additional water may be passed forwardthrough column 30 for softening purposes, then in reverse throughcolumns 50 and 40 as a rinse. in Figs. 1 to 11 and the accompanyingdescription, the term forward is used to denote flow in an upwarddirection, while reverse denotes flow downwardly. The present process isalso capable of use if these conditions are reversed. In such a case,gas release valves will be needed above columns 40 and 50.

Fig. 5 shows in detail the main operational steps of the presentprocess. A plating solution inlet 42 and plating solution outlet 52 areprovided, the latter returning regenerated plating solution to theplating bath. It is to be understood that the process, while ordinarilyof a continuous nature entailing constantly circulating the platingsolution, may be of a batch type operation. The spent electroless nickelplating solution from the plating tank 60 is passed forward throughcolumn 40, line 43, and forward through column 50 and on through line53, filter 51 and returned through outlet 52 to the plating tank 60. Thefilter 51 acts to retain any of the cation exchange resin which may beentrained with the regenerated plating solution. Carbon may also beemployed in filter 51 to remove organics from the plating solution. Thepresence of a nickel complexing agent, such as sodium citrate, in thespent plating solution will result in an exchange of the nickel ions ofthe cation exchange material for the sodium component of the sodiumcitrate forming nickel citrate as in Equation 2 above. The amount ofnickel removed from the exchange material is dependent upon and roughlyequivalent to the amount of free or sodium citrate in the spent platingsolution. When, as explained above, sufficient nickel is present in thevarious columns to provide for citrate saturation in terms of nickel,the ratio of equivalents of citrate-to-nickel is automaticallymaintained at slightly above a one-to-one ratio. This ratio ismaintained until the supply of nickel in column 40 is considerablydepleted. This ratio range has been found to be optimum for electrolessnickel plating since high efiiciency is provided with an absence ofplate roughness.

Before the nickel content of a cation exchange column 50 is excessivelydepleted, tap water from valve 2$ (Fig. 6.) is passed through column 30for softening, and then forward through columns 40 and 50 to rinse thenickel citrate out of the column. The resultant rinse solution may beused to replace evaporation losses by the immediate addition thereof tothe plating solution already passed through the column or by use foradditions to the treatment tanks. The rinse solution may also be used todissolve the hypophosphite to be added to the plating solution.

Figs. 7 through 11 show a method of recharging columns 30 and 40 withsodium. These stepsact to strip the ion exchange material of calcium andother foreign ions, replacing them with sodium ions. In Fig. 7, tapwater from valve 29 is softened by passing it through line 31 and column30 into tank 37 to prepare, typically, a saturated solution of, sodiumchloride. This step is the same as the step illustrated in Fig. 2,except for substitution of a sodium salt for the nickel salt. The sametank 37 may be used for this operation, although it is generally moreconvenient to have separate tanks available for storing both the sodiumand nickel salt solutions.

Fig. 8 illustrates the step of passing the concentrated sodium saltsolution slowly in reverse through column 30 and out to waste through asewage outlet 33.

Fig. 9 shows column 30 being rinsed forwardly with tap water from valve29, which rinse water is disposed in sewage outlet valve 36. Rinsing isdone forwardly in Fig. 9. An excess of sodium salt solution is used inFig. 8 in order to simplify the apparatus and avoid leaving foreigncations from the tap water at the wrong end of column 30. The next stepin charging columns 3% and 40is to pass the concentrated sodium saltsolution slowly in reverse through column 49 and lines 44, 41 and 39 tothe sewage outlet valve 36. As seen in Fig. 11, tap water is then passedforward through column 30 for softening purposes, then in reversethrough column 40 to rinse the sodium salt solution and the displacedcations from the column and to discard the effiuent to waste throughline 39 and sewage outlet valve 36.

When all of the nickel which is plated from the solution is to bereplaced by regeneration in column 58, the he quency of recharging thecolumn with nickel can be estimated from the column capacity and theamount of plating. The hardness of the available Water supply determinesthe frequency of recharging column 30. The calcium,'cadmium and othercations (other than sodium) are deposited from the plating solutionalmost entirely in column 40. These are then carried to Waste whendisplaced by concentrated sodium sulfate left from exchange ofconcentrated nickel sulfate addition solution during recharging Withnickel. However, a nickel concentration sufficient to completely removethese other cations would also result in excessive loss of nickel.Therefore, when more complete removal of the foreign cations is desired,a concentrated solution of a sodium salt is passed through column 40 (inreverse) to waste, after the solution containing free citrate has beenpassed forward through the same column to selectively remove the nickel.The routine treatment of used plating solution, as in Fig. 5, fulfillsthe latter requirement and completely avoids loss of nickel.

As seen in Fig. 8, a concentrated sodium salt solution is passed throughcolumn 30, in reverse, to recharge it and to replace calcium and otherdetrimental cations with sodium. Complexing agents, such as ethylenediamine tetracetate, may also be passed in reverse through one or moreof the columns 39, 40 or 50 to remove any additional foreign cations.Such agents may be passed through column 50 so long as they do not wasteexcessive amounts of nickel.

As in the past used processes of chemical nickel or cobalt plating, thepresent process may be used to plate workpieces of a metal or alloysthereof selected from the group consisting of iron, platinum, silver,gold, cobalt, palladium, nickel, aluminum, copper and titanium. Ingeneral, each of these metals must be cleaned by conventional proceduresbefore the electroless nickel or cobalt plating treatment. Thepretreatment of these various metals and their alloys is fully set outin the Brenner and Riddell patents, supra. Electroless nickel and cobaltmay also be plated directly on graphite or on glass or plastics if suchmaterials are made conductive or precoated with a catalytic metal suchas iron or aluminum.

Various types of commercially available cation exchange mediums may beused for filling columns 30, ,40 and 50. Generally, among these, thenuclear sulphonic type, the phenolic methylene sulphonic type,carboxylic and aluminum silicate type mediums may be used. Specifically,Amberlie IR-l20, produced by Rohm and Haas Company, and Dowex-SO,manufactured by the Dow Chemical Company, are examples of cationexchange resins suitable for' the use in the present process.

The preferred bath composition and characteristics are set out below inTable I.

Table I Bath composition Range Preferred N is eitratea Na hypophosphite.pH Ratio of citrate to nickel. Deposition rate mil/hr r Bath temperaturePlating time Column temperatures 0.1-1.0 0 5 212 F As required. 50170F50-140 F.

more, amino compounds such as triethylamine or ethanolamine may beemployed as the complexing agent. Potassium salts may be substituted forthe sodium salts'used in the regeneration process.

When it is desired to adapt the disclosed process to cobalt platingbaths, the pH of the solution will normally be from 8-10 as prescribedby Brenner and Riddell, supra. An excess of sodium citrate or othercomplexing agent mentioned above, or the sodium potassium tartratediscussed in Brenner et al., should be used to insure holding andcomplexing the cobalt salt in solution. Furthermore, to preventprecipitation in the ion exchange column of hydroxides, which are addedto keep the cobalt plating bath at the desired pH, the circulatingcobalt plating solution may be acidified before its entrance into thecolumn and the pH readjusted to alkaline as the regenerated solution isrecirculated to the plating bath. The parts by weight of cobalt ion,hypophosphite radical complexing radical, and water may be in the rangesexpressed in the Brenner et al., patent.

In Fig. 1, column 50 is the only essential column. Col umn 30 is usedmainly for water softening purposes and can be recharged in the standardmanner for such columns. Such column can be entirely eliminated if atank or other source of soft water is available to dissolve the nickelor cobalt salt and rinse the other columns in the reverse direction.Soft water may be obtained by passing ordinary water forward throughcolumn 40 and/or col umn 50 after they have been thoroughly rinsed.Column 40 is essentially an extension of column 50 separated to simplifyvisual control of the amount of nickel added and to permit displacementof foreign cations to waste by sodium ions only in that end of thecolumn where the nickel or cobalt is minimized and most of the foreigncations are concentrated. The hypophosphite, citrate radical andphosphite are essentially uneffccted by their passage through the cationexchange column or columns.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexampleonly and is not to be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaims.

I claim:

1. A process of treating an electroless nickel process plating solutioncontaining nickel citrate and sodium hypophosphite in which the platingprocess depletes the nickel ions in said nickel citrate and forms sodiumcitrate and hydrogen ions in said solution, said treating processcomprising passing the depleted plating solution continually through anickel-charged cation exchange medium, exchanging the nickel ions insaid medium with the sodium ions and hydrogen ions in the depletedplating solution, and recirculating the resultant nickel citratesolution to said electroless nickel plating process and maxim tainingthe ratio of equivalents of citrate-to-nickel at 1.0 to 2.0.

2. The process as set out in claim 1 in which the nickel salt of acomplexing agent is nickel citrate, the hypophosphite is sodiumhypophosphite and the ratio of equivalents of nickel-to-citrate in thecirculating solution is maintained at approximately 1:1.

3. A method of regenerating an electroless nickel plating process bathcontaining an aqueous solution of a nickel salt of a nickel complexingagent selected from the class consisting of carboxylate and aminocompounds, an alkali metal hypophosphite and impurities includingcalcium and cadmium ions, an alkali metal salt of the nickel complexingagent and hydrogen ions being formed in the plating process, comprisingcirculating said solution through a nickel-charged cation exchangematerial, exchanging nickel ions from said material with the alkalimetal ions of the complexing agent and the hydrogen ions, extractingcalcium, cadmium ions from said solution and recirculating said solutionto said plating process bath, and maintaining the ratio of equivalentsof said complexing agent-to-said nickel in the range of from 1.0 to 2.0.

4. The process of electroless plating a metal selected from the classconsisting of cobalt and nickel on a metalsurface workpiece comprisingproviding an aqueous solution of metal ions of said class and acomplexing agent selected from the class consisting of carboxylates andamino compounds in amounts such that the ratio of equivalents of saidcomplexing agent-to-said metal is in the range of from 1.0 to 2.0, andan alkali metal. hypophosphite, immersing the workpiece in said solutionfor a time sufficient to give a desired'plating thickness on saidworkpiece, circulating said solution through a cation exchange mediumcontaining cations of a metal of said class, and contacting said mediumwith said solution whereby said solution is recharged with the metalcations from said medium and maintaining said complexing agentto-saidmetal ratio.

5. The process of chemical nickel plating a metal-surfaced workpiececomprising providing an aqueous solution of nickel ions and acarboxylate complexing agent having from 2 to about 6 carbon atoms inamounts such that the ratio of equivalents of said complexingagent-tonickel is in the range of from 1.0 to 2.0, and an alkali metalhypophosphite, immersing the workpiece in said solution for a timesufficient to give a desired plating thickness of nickel metal on saidworkpiece, circulating said solution through a nickel-charged cationexchange medium, and contacting said medium with said solution wherebysaid nickel solution is recharged with nickel ions from said medium andmaintaining said complexing agent-to-said metal ratio,

6. The process of claim in which the complexing agent is a citrate andthe hypophosphite is sodium hypophosphite.

7. The process of claim 5 in which the complexing agent is a citrate,the hypophosphite is sodium hypophosphite, and the ratio of equivalentsof complexing agent-tonickel is substantially 1:1.

8. The process of chemical nickel plating a metal-surfaced workpiececomprising providing an aqueous solution of nickel ions and a complexingagent selected from the class consisting of carboxylates and aminocompounds in amounts such that the ratio of equivalents of saidcomplexing agent-to-said metal is in the range of from 1.0 to 2.0, andan alkali metal hypophosphite, immersing the workpiece in said solutionfor a time sufficient to give the desired plating thickness of nickelmetal on said object, and intermittently circulating said solutionthrough a nickel-charged cation exchange medium and contacting saidmedium with said solution whereby said solution is regenerated withnickel ions from said medium and maintaining said complexingagent-to-said metal ratio.

9. The process of chemical nickel plating a metal-surfaced workpiececomprising providing an aqueous solution of nickel ions and a complexingagent selected from the class consisting of carboxylates and aminocompounds in amounts such that the ratio of equivalents of saidcomplexing agent-to-nickel is in the range of from 1.0 to 2.0, and analkali metal hypophosphite, immersing the workpiece in said solution fora time suflicient to give the desired plating thickness of nickel metalon said object, and continuously circulating said solution through anickel-charged cation exchange medium and contacting said medium withsaid solution whereby said solution is regenerated with nickel ions fromsaid medium, intermittently passing a sodium salt solution through saidmedium 1 to remove foreign ions therefrom and intermittently rechargingsaid medium with nickel ions by passing a nickel sulfate solutiontherethrough and maintaining said complexing agent-to-said metal ratio.

10. A process of chemical nickel plating an object of a metal chosenfrom the group consisting of iron, platinum, silver, gold, cobalt,palladium, aluminum, titanium, copper and nickel, comprising the stepsof exposing said object to an aqueous solution of nickel ions and acarboxylate complexing agent having from 2 to about 6 carbon atoms inamounts such that the ratio of equivalents of said complexingagent-to-said metal is in the range of from 1.0 to 2.0, and an alkalimetal hypophosphite for a time suiiicient to give a predeterminedplating thickness of nickel metal, continually circulating the aqueoussolution through a nickel complexing cation exchange medium, saturatingthe nickel complexing agent solution with nickel cations, intermittentlyflowing a sodium salt solution through said cation exchange medium toremove foreign cations from the exchange medium, and intermittentlycountercurrent flowing a nickel sulfate solution through said cationexchange medium to recharge said medium with nickel cations.

11. A method of regenerating an electroless nickel and cobalt platingbath containing an aqueous solution of metal ions selected from theclass of cobalt and nickel, a complexing agent selected from the classconsisting of carboxylates and amino compounds, and an alkali metalhypophosphite, comprising circulating said aqueous solution through acation exchange column charged with metal ions of said class to replacethe metal ions expended in the plating process and maintaining the ratioof equivalents of said complexing agent-to-said metal ions in the rangeof from 1.0 to 2.0.

12. A process of treating an electroless nickel and cobalt processplating solution containing metal ions sc lected from the classconsisting of cobalt and nickel, a complexing agent selected from theclass consisting of carboxylates and amino compounds, and an alkalimetal hypophosphite, in which the plating process depletes the nickeland cobalt cations in said solution, forms hydrogen ions, and leavesalkali metal cation salts in said solution, said treating processcomprising passing the depleted plating solution through a cationexchange medium charged with metal ions of said class, exchanging themetal ions in said medium with the alkali metal ions and hydrogen ionsin the depleted solution, recirculating the resultant solution to saidelectroless nickel plating process, and maintaining the ratio ofequivalents of said complexing agent-tosaid metal ions in the range offrom 1.0 to 2.0.

Brenner et a1 Dec. 5, 1950 Spaulding Dec. 13, 1955 It) OTHER REFERENCESCosta: Industrial and Engineering Chemistry, vol. 42, N0. 2, February1950, pp. 308-311. 5 Kunin et 211.: Ion Exchange Resins (1950), JohnWiley & Son, Inc., N. Y., pp. 135, 139.

Paulson et al.: Plating, vol. 40, N0. 9, September 1953, pp. 1005-1009.

1. A PROCESS OF TREATING AN ELECTROLESS NICKEL PROCESS PLATING SOLUTIONCONTAINING NICKEL AND SODIUM HYPOPHOSPHITE IN WHICH THE PLATING PROCESSDEPLETES THE NICKEL IONS IN SAID NICKEL CITRATE AND FORMS SODIUM CIRATEAND HYDROGEN IONS IN SAID SOLUTION, SAID TREATING PROCESS COMPRISINGPASSING THE DEPLETED PLATING SOLUTION CONTINUALLY THROUGH ANICKEL-CHARGED CATION EXCHANGE MEDIUM, EXCHANGING THE NICKEL IONS INSAID MEDIUM WITH THE SODIUM IONS AND HYDROGEN IONS IN THE DEPLETEDPLATING SOLUTION, AND RECIRCULATING THE RESULTANT NICKEL CITRATESOLUTION TO SAID ELECTROLESS NICKEL PLATING PROCESS AND MAINTAINING THERATIO OF EQUIVALENTS OF CITRATE-TO-NICKEL AT 1.0 TO 2.0.